The present invention relates to a pneumatic radial tire having an improved carcass structure, in which steering performances are improved without increasing the tire weight.
1. Field of the Invention
In general, a pneumatic radial tire, especially a passenger radial tire is provided with a carcass made of one or two plies of radially arranged cords. In the case of one carcass ply, usually, the ply is turned up around bead cores from the inside to outside of the tire to be secured thereto.
In case of two carcass plies, 1) both of them are turned up around bead cores, or 2) one of them is turned up around bead cores and the other is turned down around bead cores from the axially outside. In either case, by making a bead apex out of high-rigidity rubber and increasing the carcass ply turnup height, the tire sidewall portions are provided with rigidity so as to increase the lateral stiffness of the tire and to obtain the desired steering performance. The steering performance is however, a contradictory requirement to tire weight reduction. PA0 In order to improve steering performances, if, as usual, the carcass ply number and the turnup heights thereof are increased, and/or the bead apex height is increased, the tire weight inevitably increases. To reduce the tire weight, by decreasing the carcass ply turnup height and/or bead apex height the steering performances are correspondingly deteriorated. PA0 Thus, it is very difficult to satisfy both of the requirements in a tire, whereas the recent improvements in vehicle performance and fuel consumption greatly require a light weight tire. PA0 As a result, when deformed, the outer carcass cords are subjected to a tensile stress and the inner carcass cords are subjected to a compressive stress. PA0 Therefore, the carcass rigidity against bending deformation is effectively increased to improve the steering performances, without increasing the tire weight.